Hey Darrell,
Where to begin
You could try the TMP search function, as there have been a lot of posts here over the past decade on this exact subject.
A great online resource is Garen Ewing's Second Afghan War site, at:
link
Good place to start there might be on the "Regiments" page, which will give you a summary of each and every British and Indian Army regt. involved throughout the war, after which if you're so inclined you can pick and choose specific units to research.
A good easily-accessible reference work is the Osprey book "The British Army on Campaign (3) 1856-1881" also by the illustrious and indefatigable Michael Barthorp, with plates by Pierre Turner, which has 2-1/2 pages of excellent colour plates of British infantry, cavalry, and artillery from the war (the extra half-page features units who fought in South Africa during the First Boer War after being dispatched from India soon after serving in Afghainstan, with the same uniforms, equipment and weapons).
A simple Google search for specific units will also yield some substantial written and visual references. Only 3 British Cavalry regts. served in the war, and one of them, the 6th Dragoon Guards, did not participate in any of the major actions, which only leaves the 9th Lancers (dressed very much like the famous 17th Lancers in the Zulu War, though sometimes with a khaki jacket in place of the blue) and the 10th Hussars (wearing the same uniform they would soon take with them to the Sudan), both of whose uniforms, weapons and equipment can easily be found online.
On the Artillery front, the big question is whether to paint your Field Artillery and RHA crews in khaki, blue, or a mix of the two. Most contemporary photos show either all khaki or all blue, and the all blue is not really evident as having been worn much in action, however no one can say it was NOT worn in action either, so it's really up to your own taste.
One great -- or perhaps problematic -- thing about the Second Afghan War when it comes to British troops is that it was literally poised on the precipice between "old school" home service dress "redcoat" style of dress and the new "drab" khaki style. At the start of the war British and Indian units were responsible for providing their own "khaki", dying white uniforms in-house with tea, dirt, etc., but by the end of the campaign the entire army was being provided with an official khaki uniform.
At the start of the war some British infantry regts -- like the 59th -- are known to have fought in at least one battle wearing scarlet frocks and blue serge trousers
while others, like the 67th Regt., split the difference, and wore khaki tunics and blue trousers
and still others, like the 66th Berkshires, were dressed all in khaki
Then there are the two Highland Regts. present -- the 92nd Gordon Highlanders, who wore kilts and khaki tunics
and the 72nd Duke of Albany's Own Highlanders, who wore khaki tunics also, but with trews and matching tartan puttees (those puttees being the only thing missing from this incredible Tony Barton scratch-built 12" figure)
You may have also noticed in the images above that some of the British Infantry Regts were still outfitted with the old "pouch-belt" equipment, meant to go with the Enfield RML (rifled muzzle loader), while others had the more modern Valise pattern, designed for use with the breech loading Martini-Henry rifle.
For me personally, this "transitional" aspect of the appearance of the British and Indian forces is one of my own favorite things about this campaign, for researching, collecting, and gaming purposes.
In the interest of shameless self-promotion, I will also leave a LINK to my Maiwand Day blog, which has a lot of info on the Second Afghan War, including more handy links to useful sites, and a page devoted to the uniforms and equipment of the Afghan Regular Army, though lately almost all I've been posting about is building terrain
maiwandday.blogspot.com
I've been a big fan of your Medieval painting and modelling skills for a long time, and wish you the very best of luck with your efforts on the Second Afghan War front!